Polymeric membranes have gained an important place in pressure driven separation technology and are used in a broad range of applications. Because of their control of the permeation of chemical species through the material, they play crucial roles in various technological processes. Despite all their advantages, continuous operation of these membranes are hindered by fouling phenomena such as deposition of retained particles, colloids, macromolecules, salts, etc., at the membrane surface, inside pores or at pore walls, resulting in a reduction in the permeation/flux from the initial rate [1-3].
Fouling starts in the form of “concentration polarization” where concentrations of rejected components build up near a membrane surface. This can increase the effective osmotic pressure across the membrane, cause a gel-layer of macromolecules, lead to precipitation of sparingly-soluble salts, or initiate deposition of a particulate cake near the membrane surface [4, 5]. In its earliest stage, fouling is reversible, as an initial permeation/flux can be often be restored by stopping filtration, or performing light flushing, such as applying back pressure. As the concentration of the rejected materials builds up, the membrane may become more tenaciously fouled, resulting in a much harder-to-reverse flux decline that requires more complex cleaning to reverse, and in fact may become irreversible. When flux decline occurs, hydraulic resistance is increased as a cake/fouled layer exerts additional resistance known as “cake resistance” [6, 7]. To address fouling, different types of pretreatment and cleaning protocols have been developed and used by specific process industries. In general, flux decline increases energy consumption (e.g., in order to maintain a targeted fluid throughput) and limits membrane lifetime due to the irreversible component as mentioned previously [1, 2]. Hence, much effort has been applied in the field of membrane technology to mitigate fouling.
Membrane separation processes, along with fouling, are predominantly surface phenomena [3, 8]. Numerous studies have been conducted in modifying the surface of membranes to reduce fouling. Among these efforts, many have aimed at changing surface energy, such as, increasing hydrophilicity of the membrane, since many fouling species are suspected of favoring adsorption on hydrophobic surfaces [3]. Adsorption of various surfactants and polymers has been utilized to increase the hydrophilicity of the membrane [9-11]. Also, much work has been done on coating polyvinyl alcohol (PVA) and polyethylene glycol (PEG) based polymers onto the membrane surface [12-14]. Surface grafting of PEG-based polymers and hydroxyethyl methacrylate are among the often-used techniques to modify the membrane surface [15-17]. In addition, various plasma treatments using CO2, N2, O2 and UV have been utilized to increase hydrophilicity of the membrane surface [18-21]. However, wider application of all these processes has been limited, as many are done under hazardous conditions. In addition, grafting and coating tends to be impermanent and, most of these techniques are expensive to do at an industrial scale [3, 22].